Installation from source: Needs MPFR and GMP libraries

either from the above web pages, or
much more conveniently from your Linux distribution package system:

Debian, Ubuntu, etc . . . . . . . .: sudo apt-get install libmpfr-dev

Fedora, Redhat, CentOS, (open)SuSE: sudo dnf install mpfr-devel

Rmpfr provides S4 classes and methods for
arithmetic including transcendental ("special") functions for
arbitrary precision floating point numbers.
To this end, it interfaces to
the LGPL ed MPFR (Multiple Precision Floating-Point Reliable) Library which itself is based on the GMP (GNU Multiple Precision) Library.

The (Swiss mirror of the)
CRAN page of Rmpfr,
Rmpfr CRAN page
contains the reference manual (help pages of all functions), vignettes
(in pdf) and further relevant information about the package.

In 2012, Rmpfr is also interfacing to the
R package gmp. gmp implements exact
arithmetic for large integers and rationals, the former also "modulo m",